The present invention relates to systems and methods for locating emitters in a prescribed geographic area and more particularly to locating mobile cellular telephones operating within a service area.
In cellular telephone systems, cellular base stations are deployed throughout the region (service area) where service is provided to cellular telephones. A cellular telephone communicates with one or more base stations. The cellular telephone infrastructure coordinates the transmissions of the mobile cellular telephones. Cellular base stations send commands to the cellular telephones directing them to transmit at certain times and at certain frequencies. When a base station initiates communication with a cellular telephone, it sends the cellular telephone a mobile station control message with a page order on its forward control channel. The cellular telephone sends a page response message on the reverse control channel. After a call is setup and the cellular telephone and the cellular base station are communicating on the reverse and forward voice channels respectively, the cellular base station may send a mobile station control message to the telephone on the forward voice channel to order the telephone to a new frequency, change its power, or command it to respond to verify it is still communicating. The cellular telephone acknowledges receipt of the message by returning an order confirmation to the cellular base station on the reverse voice channel. For the AMPS protocol, an order confirmation message is a digital signal that FSK modulates an RF carrier for 54.4 ms and produces a 30-kHz bandwidth RF signal. There is an existing protocol to communicate with the cellular telephone and this protocol is used to make the cellular telephone perform prescribed actions and respond with deterministic digital messages.
Typical base stations use six directional receive antennas, three pairs of diversity antennas each pair pointing in one of three different directions or sectors. The use of diversity along with directional antennas improves the received signal-to-noise ratio (SNR) by approximately 8 dB.
Call authorization and interfacing to the public switched telephone network is performed by the mobile switching center. When a mobile cellular telephone moves about in the service area, the call is handed off to other base stations as coordinated by the mobile switching center.
Prior art location systems deployed expensive hardware to locate each cellular telephone operating in the service area. They used wideband, high-dynamic range receivers with sub-band tuners to isolate each reverse control channel. There are 21 reverse control channels. Since the cellular telephone's transmission must be received at multiple cell sites, all 21 reverse control channels must be monitored and measured continuously by the equipment at the cell sites. To capitalize on the improvement provided by diversity and sectorized antennas, prior art systems must continuously monitor the 21 reverse control channels from six receive antennas; and consequently require a considerable number of receivers to intercept the transmissions from cellular telephones. Prior art systems have located all cellular telephones in an area and placed their locations in a database. Location applications then query the database to retrieve the location of a particular cellular telephone.
Prior art location systems forwarded to a central processor a representation of the 21 reverse control channels using a 1.536 Mbps T1 channel. A T1 channel is capable of carrying 24 voice grade channels. One encoding technique used a quantization of 1 bit per sample. To take advantage of the sectorized and diversity receivers, or to reduce the quantization noise by increasing the number of bits per sample, additional T1's are required which has the drawback of additional operating cost.
A further disadvantage of the prior art is that geolocation systems using angle-of-arrival techniques must also deploy specialized antennas. These antennas are in addition to the ones used by the cellular base station. The installation of additional antennas along with the requirement of receiving authorization from local zoning boards for installing the antennas makes angle-of-arrival systems costly.
In addition to the above background, the following references are useful in understanding the environment of the present invention.
J. R. Treichler and M. G. Larimore, "New Processing Techniques Based on the Constant Modulus Adaptive Algorithm, " IEEE Trans. Acoust., Speech, Signal Processing, vol. ASSP-31, pp. 349-472, Apr. 1983 PA0 J. R. Treichler and Brian G. Agee, "A New Approach to Multipath Correction of Constant Modulus Signals," IEEE Trans. Acoust., Speech, Signal Processing, vol. ASSP-31, pp. 459-471, Apr. 1983 PA0 Ohgane et al., "An Implementation of a CMA Adaptive Array for High Speed GMSK Transmission in Mobile Communications," IEEE Trans. on Vehicular Tech., vol. 42, no. 3, pp. 282-288, Aug. 1993 PA0 B. Widrow, J. M. McCool, and M. Ball, "The Complex LMS Algorithm,"Proc. IEEE, vol. 63, pp. 719-720, Apr. 1975 PA0 Ralph Schmidt, "Multiple Emitter Location and Signal Parameter Estimation," IEEE Trans. Antennas and Propagation, pp 276-280, Mar. 1986